The present invention relates to a method of measuring a timber piece according to the preamble of claim 1.
The invention also relates to an arrangement for measuring a timber piece according to the preamble of claim 13.
In particular, this kind of method and arrangement are applied to adjusting measurement of a timber piece in a wood handling machine and to calibrating wood measuring devices known per se.
In modern mechanical timber harvesting, in particular in timber harvesting according to what is known as “a cut-to-length method”, the demands set on the accuracy of the measurement of timber piece dimensions are constantly increasing. One reason for this is that nowadays most of the timber trade is based on measurement data automatically collected by a wood handling machine. In that case, legislation also lays down certain requirements for the measurement accuracy in timber trade. On the other hand, the benefits of accurate measurement become evident in timber harvesting based on the cut-to-length method when a whole tree trunk to be handled has to be evaluated in view of the optimal utilization of raw material after felling.
Usually, at least the diameter and length of a tree trunk or pieces to be cut from it must be measured in mechanical timber harvesting. The measurement is carried out with a certain accuracy and allowed uncertainty of measurement.
During harvesting, the diameter of a timber piece is generally measured by means which are arranged against its surface and follow it, for example by measuring the position of the feeding and/or delimbing means in the wood handling machine with respect to the frame of a harvesting head. In the length measurement, it is common to use a kind of measuring wheel which is pressed against the tree trunk and rolls along it as the tree trunk moves in the wood handling machine. Thus the measuring wheel measures the distance it travels along the outer surface of the tree trunk. The methods described above are based on the assumption and measuring principle that the tree trunk is constantly in contact with the means used in measuring.
On the other hand, measuring methods based on contact free techniques, for example, are also available. Such measuring methods may be, for example, optical or based on the use of microwaves or ultrasound. In addition to the mere outer dimensions of a timber piece, some of the above-mentioned measuring methods may also be able to determine the quality of timber or values of variables describing other properties.
Representatives of the wood processing industry, in particular, have lately expressed wishes that quality properties essential to wood processing be measured immediately in connection with mechanical timber harvesting. It would be interesting, for example, to determine knottiness, solids content, growth rate, age and, in particular, factors that deteriorate timber quality, such as skewness, decay or other faults.
It is, however, common to all measuring methods used that their accuracy needs to be monitored constantly. For example, measuring devices used in measuring the diameter and length have to be calibrated from time to time.
According to the prior art, the measuring device of a wood handling machine is calibrated in a labour-intensive manner using specific calibration scissors and a measuring tape. In practice, calibration of the diameter and/or length comprises at least the following steps:
First, a sufficiently large lot of trees are cut into timber pieces of a desired length by a wood handling machine in each working area. After this, the operator of the wood handling machine stops the machine and goes to measure the timber pieces, being simultaneously aware of their accurate order on the ground. The length and diameter of the timber pieces included in a calibration sample are typically measured at as many points as necessary, the former by a measuring tape, for example, and the latter by a large slide gauge.
Nowadays the diameter is generally measured by electronic calibrating scissors that are specially designed for this purpose and comprise means for storing measurement data and conventionally also a display for immediately showing the diameter data to the person carrying out calibration. According to the prior art, such calibrating scissors comprise a measurement tape known per se for measuring the length and determining the right point for measuring the diameter. Furthermore, such electronic calibrating scissors often inform the person carrying out the calibration of the current length position where the respective diameter is measured.
Any deviation of the cross section of the tree trunk to be measured from the ideal circular shape, in particular, and random errors dependent on the person carrying out measurement cause errors in the diameter measurement of tree trunks. The person carrying out the calibration measurement may also confuse two or more timber pieces to be calibrated with each other. In that case, the calibration result may include considerable errors. Calibration measurements are also prone to errors because they are usually performed in poor weather and lighting conditions. Timber pieces may also roll to a place different from the one where the machine operator placed them.
The result of the diameter measurement is naturally also affected by the fact how strongly the person carrying out the measurement presses the jaws of the measuring scissors against the tree trunk, for instance. The stronger the scissor jaws are pressed, the smaller the diameter result obtained. To take the deviation of the tree trunk cross section from the circular shape into account, two diameters are usually “cross-measured” at each measuring point. In the case of a trunk lying on the ground, cross-measured diameters measured substantially perpendicularly with respect to each other are seldom obtained, although this is usually desirable.
The fact that the diameter is to be determined ocularly in a perpendicular direction with respect to the longitudinal axis of the timber piece may easily also cause errors in the diameter result. The more this direction deviates from the perpendicular direction, the higher the diameter value. It should also be noted that the diameter result is always related to the position of the measuring point in the timber piece's longitudinal direction. In other words, each diameter value of a tree trunk has a respective length value. It should further be noted that the diameter of primary interest is the one measured from under the tree bark. However, the real diameter of a timber piece cannot always be measured from below the bark, at least not in daily work, but the proportion of bark in the diameter is evaluated by specific bark parameters based on research. The accuracy provided by these parameters is naturally limited.
In the length measurement of tree trunks or timber pieces, several other disadvantages also cause measurement errors. In particular, it should be kept in mind that the timber pieces to be measured more or less have the shape of a truncated cone. However, this fact is unfortunately often neglected in the prior art. In other words, when a measuring device is calibrated, the length of the outer surface of a timber piece is often accidentally measured at some point of the timber piece surface. Thus the measuring tape may be arranged in parallel with the surface of the truncated cone-shaped tree trunk, which provides a length value greater than the length of the central axis of the timber piece. The desired length is primarily estimated ocularly and free-hand by simultaneously keeping the measuring tape approximately parallel with the central axis of the timber piece.
The surface of a tree trunk is often also faulty, having for example knot stubs, or the timber piece is skewed, which may also cause errors in the measurement result. It is also possible that the ends of the timber piece have not been sawed perpendicularly with respect to the central axis of the tree trunk. In particular, the cut surface sawed to a butt log in felling may easily deviate from the perpendicular direction with respect to the central axis of the tree.
The result of the length measurement of a timber piece is naturally also affected by how strongly a measuring tape or another measuring means is stretched or pulled at the measuring moment.
According to the prior art, the quality of a timber piece is monitored and measured mainly ocularly, this being one the most important and stressing tasks of the operator of a wood handling machine. A skilled wood handling machine operator is able to measure various factors that affect the timber quality while working efficiently. However, as mechanical timber harvesting is becoming more common and introduced into new geographic locations, there will be problems of finding operators who are sufficiently skilled in quality evaluation and able to make quick decisions. There is thus a clear need for automatic quality determination of timber pieces.